1. Field of the Invention
The present invention relates to a method of preparing diamond semiconductor and particularly to a method employing corpuscular radiation to render diamond semiconducting to obtain a diamond semiconductor used in the electronics industry as a semiconductor material e.g., for an element having resistance to environment.
2. Description of the Background Art
Ion implantation is a common technique to render Si, Ge and the like semiconducting. Ion implantation, however, also leaves damage in the ion-implanted region. Accordingly, a heat treatment needs to be performed thereon. Si, Ge and the like are most stable when they have a diamond-type crystal structure, and the heat treatment would recover their original crystallinity.
Diamond may also be ion-implanted and annealed to be rendered into a semiconductor. However, as for the crystal structure of carbon, diamond under the atmospheric pressure is a metastable phase; the most stable phase is graphite, and if the same process is applied to diamond to render it semiconducting as applied to Si, Ge and the like, the diamond would be converted into graphite, not a diamond semiconductor, at the ion-implanted region.
Japanese Patent Laying-Open No. 5-24991 discloses a method of preparing a diamond semiconductor including a step of using a laser beam to perform a heat treatment on diamond with dopant ions injected therein. It suggests that the laser beam can heat the diamond to repair a crystal defect caused by the ion injection.
Japanese Patent Laying-Open No. 5-29244 discloses a method of preparing a diamond semiconductor including a step of directing dopant ions to diamond in a channeling direction thereof such as  less than 110 greater than ,  less than 100 greater than , or  less than 121 greater than  directions. It suggests that injecting a dopant in a channeling direction can result in a reduced damage to the arrangement of the carbon atoms of the diamond and that such a damage can be compensated for by a heat treatment at 1400xc2x0 C. In this method, the dopant radiated reaches a deep site of the diamond lattice.
Japanese Patent Laying-Open No. 11-100296 discloses a method of preparing a diamond semiconductor including a step of irradiating diamond heated to a temperature of 300xc2x0 C. to 1500xc2x0 C., with particles of a dopant element at a rate of 1xc3x97107 particles/cm2xc2x7sec to 1xc3x971015 particles/cm2xc2x7sec. It proposes that the temperature raised as above allows the diamond crystal to be efficiently recovered and that the diamond doped at the radiation rate range can readily recover in crystallinity.
In the prior art using a laser beam to perform a heat treatment, typically the laser beam should irradiate an area no more than 1 cm2. If the diamond has a large area, the laser beam needs to scan the diamond, which would result in a complicated process. Furthermore, the time for the heat treatment after ion implantation increases in proportion to the area of the diamond.
In the prior art employing a dopant directed in a channeling direction of diamond, a collimated ion-beam is required and precise operation is needed for the diamond""s channeling direction. Furthermore, injecting a dopant in a channeling direction with precision can result in the dopant reaching an unnecessarily deep site of the diamond lattice. Thus the dopant would be less controllable in depth.
As for the prior art controlling the temperature of diamond heated and the rate of corpuscular radiation, the present inventors have found that the diamond often fails to recover in crystallinity.
Accordingly, the present inventors have conducted researches into methods in which when applying corpuscular radiation, a diamond substrate or a thin film of diamond deposited on a substrate material is maintained at a high temperature to repair a crystal defect attributed to radiation.
An object of the present invention is to provide, based on this method, a method of preparing a diamond semiconductor capable of more reliably producing a diamond semiconductor.
Another object of the present invention is to provide a method capable of efficiently producing a diamond semiconductor by a relatively simple process.
It has been found that in a process employing corpuscular radiation for rendering diamond semiconducting, not only temperature but also the arrangement of the diamond crystal plane and the direction of the corpuscular radiation contribute to the recovery of crystallinity. Thus, suitable combinations of a temperature, an arrangement of the diamond crystal plane and a direction of corpuscular radiation have been found for recovery of the diamond crystal structure, resulting in the present invention.
The present invention is directed to a method of preparing a diamond semiconductor by irradiating, with a corpuscular ray, a diamond substrate or a thin film of diamond deposited on a base material, wherein when the corpuscular ray is applied, the diamond substrate or the thin film of diamond is maintained at a temperature of 300xc2x0 C. to 2000xc2x0 C., the surface of the diamond substrate or the thin film of diamond irradiated with the corpuscular ray forms an angle of xe2x88x9220xc2x0 to +20xc2x0 with the (001) crystal plane of the diamond substrate or the thin film of diamond, and the direction of the emitted corpuscular ray forms an angle of xe2x88x9220xc2x0 to +20xc2x0 with the  less than 001 greater than  crystal orientation of the diamond substrate or the thin film of diamond. Under such conditions, the damage caused by the corpuscular radiation can be more efficiently repaired to produce a diamond semiconductor more reliably. Preferably the corpuscular ray is emitted in a direction offset relative to  less than 001 greater than  crystal orientation by an angle of no less than 3xc2x0. Specifically, the direction of the emitted corpuscular ray preferably forms an angle of xe2x88x9220xc2x0 to xe2x88x923xc2x0 or +3xc2x0 to +20xc2x0 with the  less than 001 greater than  crystal orientation. More preferably, the angle between the direction of the emitted corpuscular ray and the  less than 001 greater than  crystal orientation is 3xc2x0 to 10xc2x0. Namely, the direction of the emitted corpuscular ray more preferably forms an angle of xe2x88x9210xc2x0 to xe2x88x923xc2x0 or +3xc2x0 to +10xc2x0 with the  less than 001 greater than  crystal orientation.
In addition, the corpuscular ray preferably irradiates the diamond substrate or the thin film of diamond having a temperature of at least 800xc2x0 C. The radiation can be formed of particles of any or a combination of group-III elements, group-V elements, Li, S, and Cl. Preferably the corpuscular ray is emitted at a rate of 1xc3x971011 particles/cm2xc2x7sec to 1xc3x971016 particles/cm2xc2x7sec. Preferably the corpuscular ray is emitted with an energy of 100 eV to 10 MeV.
In a preferred embodiment of the present invention, p and n dopant particles may be introduced in a diamond substrate or a thin film of diamond at the regions adjacent to each other in a direction of depth to prepare a pn junction device.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.